wdiff draft-ietf-sipcore-event-rate-control-04.txt draft-ietf-sipcore-event-rate-control-05.txt

Network Working Group A. Niemi
Internet-Draft K. Kiss
Updates: 3265 (if approved) Nokia
Intended status: Standards Track Nokia
Expires: January 10, 2011 S. Loreto
Expires: April 21, 2011 Ericsson
July 09,
October 18, 2010

Session Initiation Protocol (SIP) Event Notification Extension for
Notification Rate Control
draft-ietf-sipcore-event-rate-control-04
draft-ietf-sipcore-event-rate-control-05

Abstract

This document specifies mechanisms for adjusting the rate of Session
Initiation Protocol (SIP) event notifications. These mechanisms can
be applied in subscriptions to all SIP event packages.

Status of this Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions and Document Conventions . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Use Case for limiting Limiting the maximum rate Maximum Rate of notifications Notifications . 5
3.2. Use Case for setting Setting a minimum rate Minimum Rate for notifications Notifications . . 6
3.3. Use Case for specifying the average rate Specifying an Adaptive Minimum Rate of
notifications
Notifications . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. The maximum rate mechanism for Resource List Server
4. Basic Operations . . . . . . . . . . . . . . . . . . . . . . . 8
3.6. Basic Operation
4.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 8
4.2. Notifier Behavior . . . . . . 10
4. . . . . . . . . . . . . . . 9
5. Operation of the maximum rate mechanism Maximum Rate Mechanism . . . . . . . . . . . 11
4.1. 9
5.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 11
4.2. 9
5.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 11
4.3. 10
5.3. Selecting the maximum rate Maximum Rate . . . . . . . . . . . . . . . . 11
5.4. The Maximum Rate Mechanism for Resource List Server . . 12
4.4. . 11
5.5. Buffer Policy Description . . . . . . . . . . . . . . . . 13
4.4.1.
5.5.1. Partial State Notifications . . . . . . . . . . . . . 13
4.4.2.
5.5.2. Full State Notifications . . . . . . . . . . . . . . . 14
4.5. 13
5.6. Estimated Bandwidth Savings . . . . . . . . . . . . . . . 14
5.
6. Operation of the minimum rate mechanism Minimum Rate Mechanism . . . . . . . . . . . 14
5.1.
6.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 15
5.2. 14
6.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 15
6.
7. Operation of the average rate mechanism . . . . Adaptive Minimum Rate Mechanism . . . . . . . 16
6.1.
7.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 16
6.2.
7.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 17
6.3. 16
7.3. Calculating the timeout Timeout . . . . . . . . . . . . . . . . . 18
7. 17
8. Usage of "min-interval", "max-interval" the Maximum Rate, Minimum Rate and
"average-interval" Adaptive
Minimum Rate Mechanisms in a combination . . . . . . . . . . . . . 19
8. Syntax . . 18
9. Protocol Element Definitions . . . . . . . . . . . . . . . . . 19
9.1. "min-interval", "max-interval" and "average-interval"
Header Field Parameters . . . . . . . . . 20
8.1. "min-interval", "max-interval" and "average-interval"
Header Field Parameters . . . . . . . . 19
9.2. Grammar . . . . . . . . . 20
8.2. Augmented BNF Definitions . . . . . . . . . . . . . . . . 20
8.3.
9.3. Event header field usage Header Field Usage in responses Responses to the NOTIFY
request . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10.
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. 21
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
12.
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1.
13.1. Normative References . . . . . . . . . . . . . . . . . . . 22
12.2.
13.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 23

1. Introduction

The SIP events framework [RFC3265] defines a generic framework for
subscriptions to and notifications of events related to SIP systems.
This framework defines the methods SUBSCRIBE and NOTIFY, and
introduces the concept of an event package, which is a concrete
application of the SIP events framework to a particular class of
events.

One of the things the SIP events framework mandates is that each
event package specification defines an absolute maximum on the rate
at which notifications are allowed to be generated by a single
notifier. Such a limit is provided in order to reduce network load.

All of the existing event package specifications include a maximum
notification rate recommendation, ranging from once in every five
seconds [RFC3856], [RFC3680], [RFC3857] to once per second [RFC3842].

Per the SIP events framework, each event package specification is
also allowed to define additional throttle mechanisms which allow the
subscriber to further limit the rate of event notification. So far
none of the event package specifications have defined such a
mechanism.

The resource list extension [RFC4662] to the SIP events framework
also deals with rate limiting of event notifications. The extension
allows a subscriber to subscribe to a heterogeneous list of resources
with a single SUBSCRIBE request, rather than having to install a
subscription for each resource separately. The event list
subscription also allows rate limiting, or throttling of
notifications, by means of the Resource List Server (RLS) buffering
notifications of resource state changes, and sending them in batches.
However, the event list mechanism provides no means for the
subscriber to set the interval for the throttling; it is strictly an
implementation decision whether batching of notifications is
supported, and by what means.

This document defines an extension to the SIP events framework
defining the following three "Event" Event header field parameters that allow
a subscriber to set a Minimum, maximum, a Maximum minimum and an Average adaptive minimum rate
of event notifications generated by the notifier:

min-interval: specifies a minimum notification time period (maximum
rate) between two notifications, in seconds.

max-interval: specifies a maximum notification time period (minimum
rate) between two notifications, in seconds.

average-interval: specifies an average cadence at which
notifications are desired, maximum notification time
period (adaptive minimum rate) between two notifications, in
seconds.

The requirements and model are further discussed in Section 3. All
these mechanisms are simply timer values that indicate the minimum,
maximum and average maximum time period allowed between two
notifications. As a result of these mechanisms, a compliant notifier
will adjust the rate at which it generates notifications.

These mechanisms are applicable to any event subscription, both
single event subscription and event list subscription.

2. Definitions and Document Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119] and
indicate requirement levels for compliant implementations.

Indented passages such as this one are used in this document to
provide additional information and clarifying text. They do not
contain normative protocol behavior.

3. Overview

3.1. Use Case for limiting Limiting the maximum rate Maximum Rate of notifications Notifications

A presence client in a mobile device contains a list of 100 buddies
or presentities. In order to decrease the processing and network
load of watching 100 presentities, the presence client has employed a
Resource List Server (RLS) with the list of buddies, and therefore
only needs a single subscription to the RLS in order to receive
notification of the presence state of the resource list.

In order to control the buffer policy of the RLS, the presence client
sets a maximum rate ("min-interval" parameter), i.e. a minimum time
interval between two notifications. Alternatively, the presence
client could set the maximum rate for the resource list via a list
manipulation interface, e.g., using the XML Configuration Access
Protocol (XCAP) [RFC4825]. The RLS will buffer
notifications that do not comply with the maximum rate and batch all
of the buffered state changes together in a single notification only
when allowed by the maximum rate. The maximum rate applies to the
overall resource list, which means that there is a hard cap imposed
by the maximum rate to the number of notifications the presence
client can expect to receive. For example, with a "min-interval" of
20 seconds, the presence application can expect to receive a
notification at a minimum of no more frequently than every 20 seconds.

The presence client can also modify the "min-interval" parameter
during the lifetime of the subscription. For example, if the User
Interface (UI) of the application shows inactivity for a period of
time, it can simply pause the notifications by setting the "min-
interval" parameter to the subscription expiration time, while still
keeping the subscription alive. When the user becomes active again,
the presence client can resume the stream of notifications by re-
subscribing with a "min-interval" parameter set to the earlier used
value. Application of the mechanism defined by RFC 5839 [RFC5839]
can also eliminate for the presence client to receive transmission of a (full-state) notification
carrying the latest resource state after to the presence client after a
subscription refresh.

3.2. Use Case for setting Setting a minimum rate Minimum Rate for notifications Notifications

A location application is monitoring the movement of a target. In
order to decrease the processing and network load, the location
application has made a subscription with a set of location filters
[I-D.ietf-geopriv-loc-filters] that specify trigger criterias, for
example, criteria, e.g. to
send an update only when the target has moved at least n meters.
However, the application is also interested in receiving the current
state periodically even if the state of the target has not changed
enough to satisfy any of the trigger criteria, i.e. e.g. has not moved at
least n meters within the period.

In order to control the actual state, the location application sets a
minimum rate ("max-interval" parameter), i.e. a maximum time interval
between two notifications.

The location application can also modify the "max-interval" parameter
during the lifetime of the subscription. When the subscription to
the movement of a target is made, the notifier may not have the
location information available. Thus, the first notification might
be empty, or certain values might be absent. An important use case
is placing constraints on when complete state should be provided
after creating the subscription. The "max-interval" parameter
indicates the time to the notifier when a complete state information the maximum amount of time that should be notified.
allowed to elapse between NOTIFY requests containing complete state
information. Once state is acquired and the second notification is
sent, the subscriber updates or changes the "max-
interval" "max-interval" parameter
to a more sensible value. This update can be performed in the 200 OK
response to the NOTIFY request that contains the complete state
information.

3.3. Use Case for specifying the average rate Specifying an Adaptive Minimum Rate of notifications Notifications

The previous mechanisms introduce minimum rate mechanism introduces a static and instantaneous rate
control. However
control without the functionality to increase or decrease the
notification frequency adaptively. However, there are some
applications that would work better with an adaptive minimum rate
control. This section illustrates the tracking scenario.

A tracking application is monitoring the movement of a target. In
order to decrease the processing and network load, in the application, the tracking
application wants to make a subscription that dynamically increases decreases
the interval between minimum rate of notifications if the target has sent out several
notifications recently.

In order However, if there have have been only few
recent notifications by the target, the tracking application wants
the minimum rate of notifications to set increase.

The application sets an adaptive minimum rate control, the application defines a
average cadence ("average-interval" parameter) at which notifications
are desired.
parameter), i.e. an average maximum time interval between two
notifications. The "average-interval" parameter value is used by the
notifier to dynamically calculate the actual maximum time allowed ("timeout"
parameter) between two notifications. In order to dynamically
calculate the maximum, maximum time, the Notifier notifier takes into consideration the
frequency at which notifications have been sent recently.

This type of In the
adaptive minimum rate control allows mechanism the notifier to dynamically can increase or decrease
the Notification frequency. notification frequency compared to minimum rate mechanism based
on the recent number of notifications sent out in the last period.

The tracking application can also modify the "average-interval"
parameter during the lifetime of the subscription.

3.4. Requirements

REQ1: The subscriber must be able to set the minimum time period
("min-interval" a maximum rate ("min-
interval" parameter) between two of notifications in a specific
subscription.

REQ2: The subscriber must be able to set the maximum time period
("max-interval" a minimum rate ("max-
interval" parameter) between two of notifications in a specific
subscription.

REQ3: The subscriber must be able to set an average cadence adaptive minimum rate
("average-interval" parameter) at parameter), which adjusts the minimum
rate of notifications are
desired in based on a specific subscription. moving average.

REQ4: It must be possible to apply the maximum rate, the minimum
rate and the adaptive minimum rate mechanisms all together,
or in any combination, the "min-interval", "max-interval" and "average-
interval" mechanisms in a specific subscription.

REQ5: It must be possible to use any of the different rate control
mechanisms in subscriptions to any events.

REQ6: It must be possible to use any of the different rate control
mechanisms together with any other event filtering
mechanisms.

REQ7: The notifier must be allowed to use a policy in which the
minimum time period between two notifications is
"min-interval", "max-interval" and "average-interval"
paremeters are adjusted from the value given by the
subscriber.

For example, due to congestion reasons, local policy at
the notifier could temporarily dictate a policy that in
effect increases the subscriber-configured minimum time
period between two notifications. In another example, the
notifier can decrease the proposed minimum time by the
subscriber to match it with the remaining expiry time left
for the subscription.

REQ8: The different rate control mechanisms must discuss address corner
cases for setting the time periods between two notifications.
At a minimum, the mechanisms must include discussion of address the situation resulting from a minimum, maximum or average when
the time
period which exceeds between two notifications would exceed the
subscription duration, duration and should provide mechanisms for
avoiding this situation.

REQ9: The different rate control mechanisms must be possible to be
installed,
invoked, modified, or removed in the course of an active
subscription.

REQ10: The different rate control mechanisms must allow for the
application of authentication and integrity protection
mechanisms to subscriptions invoking that mechanism.

3.5. The maximum rate mechanism for Resource List Server

When applied to a list subscription [RFC4662], the maximum rate
mechanism has some additional considerations. Specifically, the
maximum rate applies to the aggregate notification stream resulting
from the list subscription, rather than explicitly controlling the
notification of each of the implied constituent events. Moreover,
the RLS can use the maximum rate mechanism on its own to control the
rate of the back-end subscriptions to avoid overflowing its buffer.

The notifier is responsible for sending out event notifications upon
state changes of the subscribed resource. We can model the notifier
as consisting of three components: the event state resource(s), the
Resource List Server (RLS) (or any other notifier), a notification
buffer, and finally the subscriber, or watcher of the event state, as
shown in Figure 1.

+--------+
| Event |
+--------+ |Resource| +--------+
| Event | +--------+ | Event |
|Resource| | |Resource|
+---.=---+ | +---=----+
`-.. | _.--'
``-._ | _.--'
+'--'--'-+
|Resource|
| List |
| Server |
+---.----+
|
|
)--+---(
| | .------------.
|Buffer|<======'min-interval|
| | `------------'
)--.---(
|
|
.---+---.
| Event |
|Watcher|
`-------'

Figure 1: Model for the Resource List Server (RLS) Supporting
Throttling

4. Basic Operations

4.1. Subscriber Behavior

In short, the RLS reads event state changes from the event state
resource, either by creating a back end subscription, or by other
means; it packages them into event notifications, and submits them
into the output buffer. The rate at which this output buffer drains
is controlled by the subscriber via the maximum rate mechanism. When
a set of notifications are batched together, general, the way in which
overlapping resource state is handled depends on the type of the
resource state:

In theory, there are many buffer policies that the notifier could
implement. However, we only concentrate on two practical buffer
policies in this specification, leaving additional ones for
further study and out of the scope of this work. These two buffer
policies depend on the mode in which the notifier is operating.

Full-state: Last (most recent) full state notification of each
resource is sent out, and all others in the buffer are discarded.
This policy applies to those event packages that carry full-state
notifications.

Partial-state: The state deltas of each buffered partial
notification per resource are merged, a subscriber generates SUBSCRIBE
requests and the resulting
notification processes NOTIFY requests is sent out. This policy applies according to those event
packages that carry partial-state notifications.

3.6. Basic Operation RFC 3265
[RFC3265].

A subscriber that wants to limit the rate of event notification in a
specific event subscription does so by including have a "min-interval"
Event header parameter as part of the SUBSCRIBE request. The "min-
interval" value indicates the maximum, minimum time allowed between
transmission of two consecutive notifications in a subscription.

Note that the witnessed time between two consecutive received
notifications may not conform to the "min-interval" value for a
number or adaptive
minimum rate of reasons. For example, network jitter and
retransmissions may result in the subscriber receiving the event notifications with smaller intervals than the "min-interval" value
recommends.

A subscriber that wants to have a maximum notification time period in a specific event subscription
does so by including a "min-interval", "max-interval"
Event header parameter as part of the SUBSCRIBE request. The "max- or "average-
interval" value indicates the maximum time allowed between
transmission of two consecutive notifications in a subscription.

A subscriber that wants to have an average cadence for the
notifications in a specific event subscription does so by including a
"average-interval" Event header parameter field parameter(s) as part of the SUBSCRIBE
request.

A subscriber that wants to update a previously agreed event rate
control parameter does so by including the updated "min-interval",
"max-interval" or "average-interval" Event header parameter field parameter(s)
as part of a subsequent SUBSCRIBE request or a 200-class 2xx response to the
NOTIFY request. If the subscriber did not include at least one of
the "min-interval, "max-interval", or "average-interval" header field
parameters in the most recent SUBSCRIBE request in a given dialog, it
MUST NOT include an Event header field with any of those parameters
in a 2xx response to a NOTIFY request in that dialog.

4.2. Notifier Behavior

In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].

A notifier that supports the different rate control mechanisms will
comply with the value given in "min-interval", "max-interval" and
"average-interval" parameters and adjust its rate of notification
accordingly. However, if the notifier needs to lower the
subscription expiration value or if a local policy or other
implementation-determined constraint at the notifier can not satisfy
the rate control request, then the notifier can adjust (i.e. increase
or decrease) opportunely appropriately the subscriber requested rate control.

5. Operation of the maximum rate mechanism

4.1. Maximum Rate Mechanism

5.1. Subscriber Behavior

In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].

A subscriber that wishes to apply a maximum rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes a
minimum time interval between two consecutive notifications included
in the "min-interval" Event header field parameter. The value of
this parameter is an integral number of seconds in decimal.

Note that the witnessed time between two consecutive received
notifications may not conform to the "min-interval" value for a
number of reasons. For example, network jitter and
retransmissions may result in the subscriber receiving the
notifications with smaller intervals than the "min-interval" value
recommends.

A subscriber that wishes to update the previously agreed maximum rate
of notifications MUST include the updated "min-interval" Event header
field parameter in a subsequent SUBSCRIBE request or a 200-class
response to the NOTIFY request. If the Event header field of the
SUBSCRIBE request did not include the "min-interval" parameter, the
subscriber MUST NOT include an initial value of the "min-interval"
Event header field parameter in a 200-class 2xx response
to the NOTIFY request.

A subscriber that wishes to remove the maximum rate control from
notifications MUST indicate so by not including a "min-interval"
Event header field parameter in a subsequent SUBSCRIBE request or a
200-class
2xx response to the NOTIFY request.

There are two main consequences for the subscriber when applying the
maximum rate mechanism: state transitions may be lost, and event
notifications may be delayed. If either of these side effects
constitute a problem to the application that is to utilize utilizes the event
notifications, developers are instructed not to use the mechanism.

4.2.

5.2. Notifier Behavior

In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].

A notifier that supports the maximum rate mechanism MUST extract the
value of the "min-interval" Event header parameter from a SUBSCRIBE
request or a 200-class 2xx response to the NOTIFY request and use it as the
suggested minimum time allowed between two notifications. This value
can be adjusted by the notifier, as defined in Section 4.3. If the Event
header field of the SUBSCRIBE request did not include the "min-
interval" parameter, the notifier MUST ignore an initial value of the
"min-interval" Event header field parameter in a 200-class response
to the NOTIFY request, if present. 5.3.

A compliant notifier MUST reflect back the possibly adjusted minimum
time interval in a "min-interval" Subscription-State header field
parameter of the subsequent NOTIFY requests. The indicated "min-
interval" value is adopted by the notifier, and the notification rate
is adjusted accordingly.

A notifier that does not understand this extension will not reflect
the "min-interval" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.

A compliant notifier MUST NOT generate notifications more frequently
than the maximum rate allows for, except when generating the
notification either upon receipt of a SUBSCRIBE request (the first
notification), when the subscription state is changing from "pending"
to "active" state or upon termination of the subscription (the last
notification). Such notifications reset the timer for the next
notification, even though they do not need to abide by it.
notification.

When a local policy dictates a maximum rate for notifications, a
notifier will not generate notifications more frequently than the
local policy maximum rate, even if the subscriber is not asking for
maximum rate control. The notifier MAY inform the subscriber about
such local policy maximum rate using the "min-interval" Subscription-
State header field parameter included in the subsequent NOTIFY
requests.

Retransmissions of NOTIFY requests are not affected by the maximum
rate mechanism, i.e., the maximum rate mechanism only applies to the
generation of new transactions. In other words, the maximum rate
mechanism does not in any way break or modify the normal
retransmission mechanism specified in RFC 3261 [RFC3261].

4.3.

5.3. Selecting the maximum rate Maximum Rate

Special care needs to be taken when selecting the "min-interval"
value. Using the "min-interval" syntax it is possible to insist both
very short and very long intervals between notifications. For example, the maximum rate could potentially set a minimum
time value between notifications that exceeds the subscription
expiration value. Such a configuration would effectively quench the
notifier, resulting in exactly two notifications to be generated. If
the subscriber requests a "min-interval" value greater than the
subscription expiration, the notifier MUST lower the "min-interval"
value and set it to the expiration time left. According to RFC 3265
[RFC3265] the notifier may also shorten the subscription expiry
anytime during an active subscription. If the subscription expiry is
shortened during an active subscription, the notifier MUST also lower
the "min-interval" value and set it to the reduced expiration time.

In some cases it makes sense to pause the notification stream on an
existing subscription dialog on a temporary basis without terminating
the subscription, e.g. due to inactivity on the application UI. user
interface. Whenever a subscriber discovers the need to perform the
notification pause operation, it SHOULD set the "min-interval" value
to the remaining subscription expiration value. This results in
receiving no further notifications until the subscription expires or
the subscriber sends a SUBSCRIBE request resuming notifications.

The notifier MAY decide to increase or decrease the proposed maximum
rate value by the subscriber based on its local policy, static
configuration or other implementation-determined constraints. The
notifier MUST include the adjusted "min-interval" value in the
Subscription-State header field's "min-interval" parameter in each of
the NOTIFY requests. In addition, different event packages MAY
define additional constraints to the allowed "min-interval"
intervals. Such constraints are out of the scope of this
specification.

4.4.

5.4. The Maximum Rate Mechanism for Resource List Server

The notifier is responsible for sending out event notifications upon
state changes of the subscribed resource. We can model the notifier
as consisting of four components: the event state resource(s), the
Resource List Server (RLS) (or any other notifier), a notification
buffer, and finally the subscriber, or watcher of the event state, as
shown in Figure 1.

Figure 1: Model for the Resource List Server (RLS) Supporting
Throttling

In short, the RLS reads event state changes from the event state
resource, either by creating a back end subscription, or by other
means; it packages them into event notifications and submits them
into the output buffer. The rate at which this output buffer drains
is controlled by the subscriber via the maximum rate mechanism. When
a set of notifications are batched together, the way in which
overlapping resource state is handled depends on the type of the
resource state:

In theory, there are many buffer policies that the notifier could
implement. However, we only concentrate on two practical buffer
policies in this specification, leaving additional ones for
further study and out of the scope of this specification. These
two buffer policies depend on the mode in which the notifier is
operating.

Partial-state: The state deltas of each buffered partial
notification per resource are merged, and the resulting
notification is sent out. This policy applies to those event
packages that carry partial-state notifications.

5.5. Buffer Policy Description

4.4.1.

5.5.1. Partial State Notifications

With partial notifications, the notifier needs to maintain a separate
buffer for each subscriber since each subscriber may have a different
value for the maximum rate of notifications. The notifier will
always need to keep both a copy of the current full state of the
resource F, as well as the last successfully communicated full state
view F' of the resource in a specific subscription. The construction
of a partial notification then involves creating a difference of the
two states, and generating a notification that contains that
difference.

When the maximum rate mechanism is applied to the subscription, it is
important that F' is replaced with F only when the difference of F
and F' was already included in a partial state notification to the
subscriber allowed by the maximum rate mechanism. Additionally, the
notifier implementation SHOULD check to see that the size of an
accumulated partial state notification is smaller than the full
state, and if not, the notifier SHOULD send the full state
notification instead.

4.4.2.

5.5.2. Full State Notifications

With full state notifications, the notifier only needs to keep the
full state of the resource, and when that changes, send the resulting
notification over to the subscriber.

When the maximum rate mechanism is applied to the subscription, the
notifier receives the state changes of the resource, and generates a
notification. If there is a pending notification, the notifier
simply replaces that notification with the new notification,
discarding the older state.

4.5.

5.6. Estimated Bandwidth Savings

It is difficult to estimate the total bandwidth savings accrued by
using the maximum rate mechanism over a subscription, since such
estimates will vary depending on the usage scenarios. However, it is
easy to see that given a subscription where several full state
notifications would have normally been sent in any given interval set
by the "min-interval" parameter, only a single notification is sent
during the same interval when using the maximum rate mechanism, mechanism
yielding bandwidth savings of several times the notification size.

With partial-state notifications, drawing estimates is further
complicated by the fact that the states of consecutive updates may or
may not overlap. However, even in the worst case scenario, where
each partial update is to a different part of the full state, a rate
controlled notification merging all of these n partial states
together should at a maximum be the size of a full-state update. In
this case, the bandwidth savings are approximately n times the size
of the header fields of the NOTIFY request.

It is also true that there are several compression schemes available
that have been designed to save bandwidth in SIP, e.g., SigComp
[RFC3320] and TLS compression [RFC3943]. However, such compression
schemes are complementary rather than competing mechanisms to the
maximum rate mechanism. After all, they can both be applied
simultaneously, and in such a way that the compound savings are as
good as the sum of applying each one alone.

5.
simultaneously.

6. Operation of the minimum rate mechanism
5.1. Minimum Rate Mechanism

6.1. Subscriber Behavior

In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].

A subscriber that wishes to apply a minimum rate to notifications in
a subscription MUST construct a SUBSCRIBE request that includes a
maximum time interval between two consecutive notifications included
in the "max-interval" Event header field parameter. The value of
this parameter is an integral number of seconds in decimal.

A subscriber that wishes to update the previously agreed minimum rate
of notifications MUST include the updated "max-interval" Event header
field parameter in a subsequent SUBSCRIBE request or a 200-class
response to the NOTIFY request. If the Event header field of the
SUBSCRIBE request did not include the "max-interval" parameter, the
subscriber MUST NOT include an initial value of the "max-interval"
Event header field parameter in a 200-class 2xx response
to the NOTIFY request.

A subscriber that wishes to remove the minimum rate control from
notifications MUST indicate so by not including a "max-interval"
Event header field parameter in a subsequent SUBSCRIBE request or a
200-class
2xx response to the NOTIFY request.

The main consequence for the subscriber when applying the minimum
rate mechanism is that it can receive a notification even if nothing
has changed in the current state of the notifier. However, RFC 5839
[RFC5839] defines a mechanism that allows sending only an etag
instead of the full resource state in a notification if the state has
not changed.

5.2.

6.2. Notifier Behavior

In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].

A notifier that supports the minimum rate mechanism MUST extract the
value of the "max-interval" Event header field parameter from a
SUBSCRIBE request or a 200-class 2xx response to the NOTIFY request and use it
as the suggested maximum time allowed between two notifications. If the
Event header field of the SUBSCRIBE request did not include the "max-
interval" parameter, the notifier MUST ignore an initial value of the
"max-interval" Event header field parameter in a 200-class response
to the NOTIFY request, if present.

The notifier MAY decide to increase or decrease the proposed minimum
rate value based on its local policy, static configuration or other
implementation-determined constraints. A compliant notifier MUST
reflect back the possibly adjusted maximum time interval in a "max-
interval" Subscription-State header field parameter of the subsequent
NOTIFY requests. The indicated "max-interval" value is adopted by
the notifier, and the notification rate is adjusted accordingly.

A notifier that does not understand this extension, will not reflect
the "max-interval" Subscription-State header field parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.

A compliant notifier MUST generate notifications whenever when state changes
occur or when the time since the most recent notification exceeds the
value in the "max-
interval" "max-interval" parameter. Depending on the event
package and subscriber preferences indicated in the SUBSCRIBE
request, the NOTIFY request sent as a result of a max-interval
expiration MUST contain either the current full state or the partial
state showing the difference between the current state and the last
successfully communicated state.

Retransmissions of NOTIFY requests are not affected by the minimum
rate mechanism, i.e., the minimum rate mechanism only applies to the
generation of new transactions. In other words, the minimum rate
mechanism does not in any way break or modify the normal
retransmission mechanism.

7. Operation of the average rate mechanism

6.1. Adaptive Minimum Rate Mechanism

7.1. Subscriber Behavior

In general, the way in which a subscriber generates SUBSCRIBE
requests and processes NOTIFY requests is according to RFC 3265
[RFC3265].

A subscriber that wishes to apply an average adaptive minimum rate to
notifications in a subscription MUST construct a SUBSCRIBE request
that includes a
proposed an average maximum time interval between two
consecutive notifications included in a "average-interval" Event
header field parameter. The value of this parameter is an integral
number of seconds in decimal.

A subscriber that wishes to update the previously agreed average adaptive
minimum rate of notifications MUST include the updated "average-interval" "average-
interval" Event header field parameter in a subsequent SUBSCRIBE
request or a 200-
class response to the NOTIFY request. If the Event header field of
the SUBSCRIBE request did not include the "average-interval"
parameter, the subscriber MUST NOT include an initial value of the
"average-interval" Event header field parameter in a 200-class 2xx response to the NOTIFY request.

A subscriber that wishes to remove the average adaptive minimum rate control
from notifications MUST indicate so by not including a "average-interval" "average-
interval" Event header field parameter in a subsequent SUBSCRIBE
request or a
200-class 2xx response to the NOTIFY request.

The main consequence for the subscriber when applying the average adative
minimum rate mechanism is that it can receive a notification even if
nothing has changed in the current state of the notifier. However,
RFC 5839 [RFC5839] defines a mechanism that allows sending only an
etag instead of the full resource state in a notification if the
state has not changed.

6.2.

7.2. Notifier Behavior

In general, the way in which a notifier processes SUBSCRIBE requests
and generates NOTIFY requests is according to RFC 3265 [RFC3265].

A notifier that supports the average adaptive minimum rate mechanism MUST
extract the value of the "average-interval" Event header parameter
from a SUBSCRIBE request or a 200-class 2xx response to the NOTIFY request and
use it to calculate the actual maximum time allowed between two transactions notifications
as defined in Section 6.3. If the Event header field of the
SUBSCRIBE request did not include the "average-interval" parameter,
the notifier MUST ignore an initial value of the "average-interval"
Event header field parameter in a 200-class response to the NOTIFY
request, if present. 7.3.

The notifier MAY decide to increase or decrease the proposed average
time interval
"average-interval" based on its local policy, static configuration or
other implementation-determined constraints. A compliant notifier
MUST reflect back the possibly adjusted average time interval in an
"average-interval" "average-
interval" Subscription-State header field parameter of the subsequent
NOTIFY requests. The indicated "average-interval" value is adopted
by the notifier, and the notification rate is adjusted accordingly.

A notifier that does not understand this extension will not reflect
the "average-interval" Subscription-State header parameter in the
NOTIFY requests; the absence of this parameter indicates to the
subscriber that no rate control is supported by the notifier.

A compliant notifier SHOULD generate notifications whenever when state changes
occur or when the time since the most recent notification exceeds the
value calculated using the formula defined in Section 6.3. 7.3.

The average adaptive minimum rate mechanism is implemented as follows:

1) When a subscription is first created, the notifier creates a
record that keeps track of the number of notifications that have
been sent in the "period". This record is initialized to contain
a history of having sent one message every "average-interval"
seconds for the "period".

2) The "timeout" value is calculated according to the equation given
in Section 6.3. 7.3.

3) If the timeout period passes without a NOTIFY request being sent
in the subscription, then the current resource state is sent
(subject to any filtering associated with the subscription).

4) Whenever a NOTIFY request is sent (regardless of whether due to a
timeout or a state change), the notifier updates the notification
history record, recalculates the value of "timeout," and returns
to step 3.

Retransmissions of NOTIFY requests are not affected by the timeout,
i.e., the timeout only applies to the generation of new transactions.
In other words, the timeout does not in any way break or modify the
normal retransmission mechanism specified in RFC 3261 [RFC3261].

6.3.

7.3. Calculating the timeout Timeout

The formula used to vary the absolute pacing in a way that will meet
the average adaptive minimum rate requested over the period is given in
equation (1):

timeout = (average-interval ^ 2) * count / period (1)

The output of the formula, "timeout", is the time to the next
notification, expressed in seconds. The formula has three inputs:

average-interval: The value of the "average-interval" parameter
conveyed in the Subscription-State header field, in seconds.

period: The rolling average period, in seconds. The value of the
"period" parameter is chosen by the notifier, however the notifier
MUST be choose a value greater than the value of the "average-
interval" parameter. The granularity of the values for the
"period" parameter is set by local policy at the notifier. It is
an implementation decision whether the notifier uses the same
value of the "period" parameter for all subscriptions or
individual values for each subscription.

count: The number of notifications that have been sent during the
last "period" of seconds not including any retransmissions of
requests.

In case both the maximum rate and the average adaptive minimum rate
mechanisms are used in the same subscription the formula used to
dynamically calculate the timeout is given in equation (2):

timeout = MAX[min-interval, (average-interval ^ 2) * count / period] (2)

min-interval: The value of the "min-interval" parameter conveyed in
the Event Subscription-State header field, in seconds.

The formula in (2) makes sure that for all the possible values of the
"min-interval" and "average-interval" parameters, with "average-
interval" > "min-interval", the timeout never results in a lower
value than the value of the "min-interval" parameter.

In some situation it may be beneficial for the Notifier notifier to achieve an
average
adaptive minimum rate in a different way than the algorithm detailed
in this document allows. However, the Notifier notifier MUST comply with any "min-
interval"
"min-interval" or "max-interval" parameters that have been
negotiated.

8. Usage of "min-interval", "max-interval" the Maximum Rate, Minimum Rate and "average-interval" Adaptive Minimum Rate
Mechanisms in a combination

Applications can subscribe to an event package using all the rate
control mechanisms individually, or in combination; in fact there is
no technical incompatibility among them. However there are some
combinations of the different rate control mechanisms that make
little sense to be used together. This section lists all the
combinations that are possible to insert in a subscription; the
utility to use each combination in a subscription is also analyzed.

min-interval

maximum rate and max-interval: this minimum rate: This combination allows to reduce the
notification frequency rate, but at the same time assures the
reception of a notification every time the most recent
notification exceeds a specified interval.

A subscriber SHOULD choose a "max-interval" value higher than the
"min-interval" value, otherwise the notifier MUST adjust the
subscriber provided "max-interval" value to a value equivalent equal to or
higher than the "min-interval" value.

min-interval

maximum rate and average-interval: it adaptive minimum rate: It works in a similar way as
the combination above, but with the difference that the interval
at which notifications are assured changes dynamically.

A subscriber SHOULD choose a "average-interval" value higher than
the "min-interval" value, otherwise the notifier MUST adjust the
subscriber provided "average-interval" value to a value equivalent equal to
or higher than the "min-interval" value.

max-interval

minimum rate and average-interval: as both adaptive minimum rate: When using the parameters are
designed as adaptive
minimum rate mechanisms, this combination makes sense
only mechanism, frequent state changes in some corner cases. a short period
can result in no notifications for a longer period following the
short period. The addition of the minimum rate mechanism ensures
the subscriber always receives notifications after a specified
interval.

A subscriber SHOULD choose a "max-interval" value higher than the
"average-interval" value, otherwise the notifier MUST NOT consider
the "max-interval" value.

min-interval, max-interval

maximum rate, minimum rate and average-interval: this adaptive minimum rate: This
combination makes little sense to be used although not forbidden.

A subscriber SHOULD choose a "max-interval" and "average-interval"
values higher than the "min-interval" value, otherwise the
notifier MUST adjust the subscriber provided "max-interval" and
"average-interval" values to a value equivalent equal to or higher than the
"min-interval" value.

A subscriber SHOULD choose a "max-interval" value higher than the
"average-interval" value, otherwise the notifier MUST NOT consider
the "max-interval" value.

8. Syntax

9. Protocol Element Definitions

This section describes the syntax protocol extensions required for the
different rate control mechanisms.

8.1.

9.1. "min-interval", "max-interval" and "average-interval" Header Field
Parameters

The "min-interval", "max-interval" and "average-interval" parameters
are added to the rule definitions of the Event header field and the
Subscription-State header field in RFC 3265 [RFC3265] grammar. Usage
of this parameter is described in Section 4, 5, Section 5 6 and Section 6.

8.2. Augmented BNF Definitions 7.

9.2. Grammar

This section describes the Augmented BNF [RFC5234] definitions for
the new syntax elements. header field parameters. Note that we derive here from the
ruleset present in RFC 3265 [RFC3265], adding additional alternatives
to the alternative sets of "event-param" and "subexp-params" defined
therein.

event-param =/ min-interval-param
subexp-params =/ min-interval-param
min-interval-param = "min-interval" EQUAL delta-seconds

event-param =/ max-interval-param
subexp-params =/ max-interval-param
max-interval-param = "max-interval" EQUAL delta-seconds

event-param =/ average-interval-param
subexp-params =/ average-interval-param
average-interval-param = "average-interval" EQUAL delta-seconds

8.3.

9.3. Event header field usage Header Field Usage in responses Responses to the NOTIFY request

This table expands the table described in Section 7.2 of RFC 3265
[RFC3265] allowing the Event header field to appear in a 200-class 2xx response
to a NOTIFY request. The use of the Event header field in responses
other than 2xx to NOTIFY requests is undefined and out of scope of
this specification.

Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
-----------------------------------------------------------------
Event 2xx - - - - - - - - o

A UA subscriber that wishes to update the value for minimum, maximum maximum, minimum or average
adaptive minimum rate of notifications can do so by including all
desired values for the rate control "min-interval", "max-interval" and "average-
interval" parameters in an Event header field of the 200-class 2xx response to
a NOTIFY request.

Header Any of the other header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
----------------------------------------------------------------- parameters currently
defined for the Event header field by other specifications do not
have a meaning if the Event header field is included in the 2xx - - - - - - - - o

9.
response to the NOTIFY request. These header field parameters MUST
be ignored by the notifier, if present.

The event type listed in the Event header field of the 2xx response
to the NOTIFY request MUST match the event type of the Event header
field in the corresponding NOTIFY request.

10. IANA Considerations

This specification registers three new SIP header field parameters,
defined by the following information which is to be added to the
Header Field Parameters and Parameter Values sub-registry under
http://www.iana.org/assignments/sip-parameters.

Predefined
Header Field Parameter Name Values Reference
-------------------- --------------- ---------- ---------
Event min-interval No [RFCxxxx]
Subscription-State min-interval No [RFCxxxx]
Event max-interval No [RFCxxxx]
Subscription-State max-interval No [RFCxxxx]
Event average-interval No [RFCxxxx]
Subscription-State average-interval No [RFCxxxx]

(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)

10.

This specification also updates the reference defining the Event
header field in the Header Fields sub-registry under
http://www.iana.org/assignments/sip-parameters.

Header Name compact Reference
----------- ------- ---------
Event o [RFC3265][RFCxxxx]

(Note to the RFC Editor: please replace "xxxx" with the RFC number of
this specification, when assigned.)

11. Security Considerations

Naturally, the security considerations listed in RFC 3265 [RFC3265],
which the rate control mechanisms described in this document extends,
apply in entirety. In particular, authentication and message
integrity SHOULD be applied to subscriptions with this extension.

RFC 3265 [RFC3265] recommends the integrity protection of the Event
header field of SUBSCRIBE requests. Implementations of this
extension SHOULD also provide integrity protection for the Event
header field included in the 200-class 2xx response to the NOTIFY request.

Without integrity protection an eavesdropper could see and modify the
Event header field; or it could also manipulate the transmission of a
200 (OK) response to the NOTIFY request in order to suppress or flood
notifications without the subscriber seeing what caused the problem.

When the maximum rate mechanism involves partial state notifications,
the security considerations listed in RFC 5263 [RFC5263] apply in
entirety.

11.

12. Acknowledgments

Thanks to Pekka Pessi, Dean Willis, Eric Burger, Alex Audu, Alexander
Milinski, Jonathan Rosenberg, Cullen Jennings, Adam Roach, Hisham
Khartabil, Dale Worley, Martin Thomson, Byron Campen, Alan Johnston,
Michael Procter and Janet Gunn for support and/or review of this
work.

Thanks to Brian Rosen for the idea of the minimum and average adaptive
minimum rate mechanisms, and Adam Roach for the work on the averaging algorithm
for the adaptive minimum rate mechanism and other feedback.

12.

13. References

12.1.

13.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.

[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.

[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC5263] Lonnfors, M., Costa-Requena, J., Leppanen, E., and H.
Khartabil, "Session Initiation Protocol (SIP) Extension
for Partial Notification of Presence Information",
RFC 5263, September 2008.

12.2.

13.2. Informative References

[I-D.ietf-geopriv-loc-filters]
Mahy, R., Rosen, B., and H. Tschofenig, "Filtering
Location Notifications in the Session Initiation Protocol
(SIP)", draft-ietf-geopriv-loc-filters-11 (work in
progress), March 2010.

[RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
Liu, Z., and J. Rosenberg, "Signaling Compression
(SigComp)", RFC 3320, January 2003.

[RFC3680] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.

[RFC3842] Mahy, R., "A Message Summary and Message Waiting
Indication Event Package for the Session Initiation
Protocol (SIP)", RFC 3842, August 2004.

[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.

[RFC3857] Rosenberg, J., "A Watcher Information Event Template-
Package for the Session Initiation Protocol (SIP)",
RFC 3857, August 2004.

[RFC3943] Friend, R., "Transport Layer Security (TLS) Protocol
Compression Using Lempel-Ziv-Stac (LZS)", RFC 3943,
November 2004.

[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

[RFC5839] Niemi, A. and D. Willis, "An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
Notification", RFC 5839, May 2010.

Authors' Addresses

Aki Niemi
Nokia
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland

Phone: +358 50 389 1644
Email: aki.niemi@nokia.com
Krisztian Kiss
Nokia
323 Fairchild Dr
Mountain View, CA 94043
US

Phone: +1 650 391 5969
Email: krisztian.kiss@nokia.com

Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland

Email: salvatore.loreto@ericsson.com